The present invention relates to new stabilized compositions based on halogenated polymers and more specifically on chlorinated polymers. Halogenated polymers tend to dehydrohalogenate to release hydrohalogenic acid when being processed with heat; this results in the formation of polyene double bonds which are responsible for coloring the polymers.
Chlorinated polymers, for example, especially polyvinyl chloride (PVC), dehydrochlorinate to release hydrochloric acid when processed with heat. Numerous additives have been recommended to deal with this problem, on the one hand to limit the formation of hydrochloric acid or to fix it and on the other hand to decelerate the propagation of polyene double bonds. Reference may be made, for example, to the work The Stabilization of Polyvinyl Chlorides by F. Chevassus (Amphora Publications, 1957).
Organic zinc and cadmium compounds, particularly in combination with organic compounds of calcium, barium or magnesium, are widely employed as basic stabilizers for chlorinated polymers. In most cases, however, these compounds need to be used in combination with other stabilizers, sometimes called secondary stabilizers. French Patent FR-A-2,297,227 describes PVC compositions stabilized effectively by organic metal salts, e.g., salts of zinc, calcium and barium, and a .beta.-diketone. French Patent FR-A-2,483,934 recommends the use of a hydrotalcite with a zinc compound, but the results were poor with respect to the color of the PVC. European Patent EP-A-63,180 proposes employing organic metal salts, a .beta.-diketone or a metal enolate of .beta.-diketone, and a hydrotalcite. The presence of hydrotalcite in these stabilizing compositions does not contribute any significant advantage with regard to yellowing during the processing of the polymer with heat.
Now, it is necessary to have a period of absence of color, or of slow color development which is as long as possible, during the processing of the chlorinated polymer with heat, especially in the case of applications in which a transparent and a colorless, or faintly colored, appearance of the shaped object are required. This is sometimes called the available working range.
The present invention provides a solution to this problem of lengthening of the working range.